Many industrial processing and manufacturing applications require the use of highly toxic fluids. The manufacture of semiconductor materials represents one such application wherein the safe storage and handling of highly toxic hydridic or halidic gases becomes necessary. Examples of such gases include silane, germane, ammonia, phosphine, arsine, stibine, hydrogen sulfide, hydrogen selenide, hydrogen telluride, and other halide compounds. As a result of toxicity and safety considerations, these gases must be carefully stored and handled in the industrial process facility. The semiconductor industry in particular relies on the gaseous hydrides of arsine (AsH.sub.3) and phosphine (PH3) as sources of arsenic (As) and phosphorus (P) in ion implantation. Ion implantation systems typically use dilute mixtures of AsH.sub.3 and PH.sub.3 at pressures as high as 1500 psig. Due to their extreme toxicity and high vapor pressure, their use, transportation and storage raise significant safety concerns for the semiconductor industry.
Looking at arsine handling as a more specific example of how an extremely toxic gas is used by the semiconductor industry, arsine is typically stored in pressurized containers at about 250 psi. The handling of arsine cylinders in production environments presents a wide variety of hazardous situations. A leak in one 140 gram cylinder of arsine could contaminate the entire volume of a 30,000 square foot building with 10 foot high ceilings to the Immediate Danger to Life and Health (IDLH) level. If the leak were large, this could happen in just a minute or two, which would mean that for many hours there would be extremely deadly concentrations in the area near the source of the spill.
An arsine container typically uses a 500 cc gas cylinder with a valve at one end. Liquid arsine pumped at about 250 psi fills the cylinder to about 20% of its capacity (about 140 grams of arsine). Once filled, the valve is closed and a safety cap is installed on the valve outlet port. The cylinder is light (about 5 pounds) and the valve is strong compared to the weight of the cylinder so that dropping the cylinder onto the valve end from 10 or 20 feet above a concrete floor will not breach the integrity of the valve or cylinder. This strength of these small cylinders eliminates the need for the valve protection that usually appears on larger gas cylinders.
An end-user that receives the container will, in a well ventilated area, remove the safety cap, install the container, usually vertically, on the end-use apparatus, and open the valve. The container then dispenses liquid or gas arsine depending on the position of the valve end. If the valve end is down, arsine liquid will be dispensed. If the valve end is up, arsine gas will be dispensed. Regardless of valve position, end-user apparatus always uses arsine in gas phase whether discharged from the cylinder as a gas or converted from liquid to gas within the end-user apparatus.
The saturation pressure of liquid arsine at room temperature (22.degree. C.) is about 250 psi. This means that any leak in the container to apparatus connections or in the end user apparatus itself will have arsine exiting to atmosphere at 250 psi. Thus, connections that remain absolutely leak tight to 250 psi or better must join all parts of the apparatus and supply container. If the end user were to first open the valve and then remove the safety plug, the entire 140 grams of arsine could spill out in as little as one or two seconds, especially if the valve end were down. Such an event could happen if someone turns the valve handle full open hard with enough torque such that the handle sticks sufficiently to mislead someone else into thinking that the valve was closed. Removal of the safety cap or disconnection of the cylinder under the mistaken belief that the valve was closed could then result in a rapid release of arsine.
In view of the serious potential for injury or death that could result from an unintended release of these fluids, the prior art discloses systems for preventing such catastrophic release of toxic fluids. U.S. Pat. No. 4,744,221 teaches the storing and the subsequent delivery of arsine by contacting arsine at a temperature of from about -30.degree. C. to about +30.degree. C. with a zeolite to adsorb arsine on the zeolite for storage. Heating then dispenses the arsine from the zeolite at an elevated temperature of up to about 175.degree. C. The method of the '221 patent imposes a disadvantageous heating requirement on the arsine delivery. One problem with heating is that the storage vessel typically has a significant heat capacity. The heat capacity of the storage vessel introduces a significant lag time in the dispensing operation. Further, heating can decompose the arsine resulting in the formation of hydrogen gas with its potential explosion hazards. Thermal decomposition of arsine also causes an undesired increase in gas pressure for the process system.
U.S. Pat. No. 5,518,528 provides a system for storage and dispensing of hydridic and halidic gases which operates at ambient temperature by using a pressure reduction to desorb toxic fluids from zeolite materials having high storage (sorptive) capacity for these gases. The '528 patent uses a dispensing assembly to provide a pressure below the interior pressure of the storage vessel. The reduced pressure desorbs the sorbate gas from the solid-phase physical sorbent medium. In order to retrieve a significant portion of the arsine off of the adsorbent, very low pressures must be used. When full, the dispensing pressure might be 600 torr. When half full it will be down to about 70 torr. Most mass flow controllers are only rated down to 150 torr operating pressure. At 150 torr 60% of the arsine on the adsorbent remains on the adsorbent. Some modifications to the customer's equipment may be necessary to install the very low pressure mass flow controllers required for utilizing more than 40% of the arsine in an adsorbent type cylinder.
Valve lock arrangements provide more direct means of limiting the flow of liquid from carrier gas storage devices. U.S. Pat. No. 4,738,693 specifically discloses the use of membrane and diaphragm elements to prevent liquid discharge in the delivery of dopines for the semi-conductor industry.
The use of tubes with multiple capillary passages presents problems of forming the capillary structure into an arrangement that is suitably connected to the container that requires the flow restriction. It is known in the art to make a multi-passage capillary assembly using hollow tubes with outer hexagonal profiles. The circular holes in such a bundle are ordered with no void space in the walls. One problem with bundling of hexagonal capillaries is that the glass is typically a low melting point lead glass. Lead glass can be easily cast or shaped through a die to form the required external hex pattern. However, the low melting point of the lead glass results in a structure that is stiff and easy to shatter. Generally, the lower the melting temperature of a glass the greater the modulus of elasticity (stiffness) of that glass. Lead glass with its low melting temperature has a somewhat higher modulus of elasticity than higher melting point glasses. Therefore, for a given strain (deformation), it has higher internal stresses and is accordingly more likely to fracture. Lead glass has a further problem of chemical erosion. But the biggest disadvantage to this approach is the resultant final shape of the assembly which is a hex. Since the ends of the multi-capillary need to be attached to other parts in a gas chromatograph, the hex shape causes difficulties in getting compression type fittings to interface.
The problem of making connections to capillary structures is not a trivial one. The fine diameters of tubing and the low tensile strength of capillary column materials, such as fused silica, makes the arrangement of capillary columns and of capillary connectors for the capillary tubes especially difficult. Although many methods and procedures for making such connections are possible the connections generally require bonding to a conduit that has a circular cross-section. A suitable connection arrangement is described in U.S. Pat. No. 5,692,078.
The obvious solution is to make the outer cross section of the multi-capillary a circle for a more compatible fit to conventional compression fittings. This approach though confronts a mathematical problem that nobody has solved and that is: small circles in a larger circle do not pack in a uniform manner. This problem has presented itself in many different forms over the last several hundred years in stranded steel cables, in electrical conduits, etc. Simply put, circles packed together do not want to form an outer shape of a circle--circles packed together with the proper number of elements form hex shaped outlines.
It is a broad object of this invention to limit the release of toxic gases in the event of a valve or conduit failure.
It is an object of this invention to provide a multi-passage capillary assembly that has high ductility and a cross section compatible with the necessary fittings for sealing fluid flow through the capillary.
It is a further object of this invention to provide a multi-passage capillary assembly that provides a high degree of uniformity in the individual cross sections of the multiple capillaries and has an outer cross section of the assembly that is compatible with the necessary fittings for sealing fluid flow through the capillary passages.
A yet further object of this invention is to provide a discharge system that constrains the flow of gas during normal operation as well as during any kind of valve mishandling or valve failure.
A specific object of this invention is to provide safeguards for the delivery of arsine.